1. Field of the Invention
The present invention relates generally to electronic display equipment and more particularly to LED backlighting for LCD displays.
2. Description of the Prior Art
Liquid crystal displays (LCDs) use backlighting to affect the brightness, contrast, and other aspects of the viewing experience. A typical LCD panel has crystalline material suspended in a liquid medium between glass plates. An array of semiconductor switches in the LCD module applies electric fields to the crystalline material to align the crystalline material to either block light or permit light to pass through the display. The crystalline material thus acts as a shutter to control the light passing through the LCD. Because LCD displays produce no light of their own, a light source positioned to illuminate the LCD panel provides back lighting by shining light through the display.
Referring to FIG. 1, one type of traditional LCD backlight assembly 10 has a cold-cathode fluorescent tube lamp (CCFTs) 12 placed along one side of a LCD panel 14. A light diffuser plate 15 is positioned behind LCD panel 14 and tapers in thickness from a first end 16 near CCFT 12 to a second end 18. A back surface of diffuser plate 14 has a reflective coating 20. Light 22 incident to reflective coating 20 reflects into a polarizing film 24. Internal refraction causes light 22 to be reflected and eventually transmitted towards LCD panel 14 in a predetermined direction 26, thereby transmitting polarizing light emitted from CCFT 12 through LCD panel 14. Light diffuser plate 14 and polarizing film 24 evenly spread the light across the display. CCFT technology has been used extensively for many years. Compared to white LEDs, most CCFTs have a more even white spectral output that provides a more complete color gamut for the display than LED backlighting. Compared to RGB LEDs, CCFTs are deficient in red light wavelengths. However, CCFTs are considered less energy efficient than LEDs and require an inverter to transform the low voltage used for the LCD (usually 5 or 12 v) to a high voltage needed to light a CCFT. The thickness of the inverter transformer often determines the minimum thickness of the LCD display.
In the interest of energy efficiency and in minimizing overall display thickness, another method of backlighting uses a row of white LEDs placed along edges of the display to illuminate the LCD display. Similar to CCFT technology, a diffuser evenly spreads the light across the display. LED backlighting of this type has been widely used in desktop computer monitors.
In yet another type of backlighting, an array of white or RGB LEDs placed behind a diffuser positioned in back of the LCD panel illuminates the LCD display from behind. LCD displays with this backlight technology usually have the ability to dim the LEDs in the dark areas of the image being displayed, effectively increasing the contrast ratio of the display. This type of LED array backlighting has been widely used in LCD televisions.
With array-based LCD displays, open areas exist between each liquid crystal (or “pixel”) making up the array. Light from white LED or CCFT backlighting bleeds or leaks around each LCD pixel, causing a blooming effect due to unnecessary illumination around the fringes of LCD pixels. This blooming effect is sometimes referred to as “inter-pixel light leakage.”